Fred, if you have the original L400 latch, the bad news is that there's nothing out there that really fits in that molded space in terms up newer more robust door latches. But it sounds like you're pretty handy, so there are some options.
When my dad and I replaced my old latch with a new model door handle that has both a deadbolt and a locking latch (a Trimark I think), we did a bunch of
fiberglass work to re-sculpt the little molded inset on the door to make it work. The new latch is more of a square shape. But we were doing a whole bunch of body work ending in totally sanding the thing down and repainting, so in that context the mess we made modifying the door wasn't a huge deal.